Tweetdeck
Probably the most popular way to actually read and send tweets. Worth taking for a spin because it lets you sort and organize the information streaming in in different ways.

Math Tweeps
This Google Doc is a growing list of Math Teachers who are on Twitter. It helps you find people. I'm trying to add peoples' websites and blogs to the list since sometimes it's nice to learn more about the people you're hearing from.

Twitter for Newbies
This Google Doc has some great tips for using Twitter as well as links to other Twitter resources and videos. Plus, there's a list of other education leaders and Twitter chats you can tune into.